8. PROJECT SUMMARY Attention-Deficit/Hyperactivity Disorder (ADHD) is the most common neurodevelopmental disorder; however, the mechanisms underlying attention problems in individuals with the disorder are unclear. Prominent neurobiological models hypothesize that task-negative regions of the brain (i.e., default-mode network) are not adequately suppressed during task performance which culminate in increased reaction time variability (i.e., attentional lapse model). Recent evidence, however, indicates that increased reaction time variability can reflect both the failure to recruit task-positive (i.e., dorsal attention network [DAN]) regions of the brain as well as inadequate suppression of task-negative regions (i.e., default-mode network [DMN]) ? a possibility that has not been examined in children with ADHD. The current proposal seeks to evaluate the utility of a novel approach to isolating the unique contribution of DAN and DMN to reaction time variability in children with ADHD. Specifically, our aims are 1) to examine RTV related activations in the DAN and DMN in children with ADHD, 2) to compare the RTV related activations in DAN and DMN between children with and without ADHD, and 3) examine the mediating effect of brain network activity (DAN, DMN) on the relation between RTV and behavioral symptoms of the disorder (i.e., inattention, hyperactivity, impulsivity) in children with ADHD and whether these relationships are moderated by distinct neurobiologically impaired subgroups of children with ADHD (i.e., DAN impaired, DMN impaired). This proposal attempts to synthesize neurobiological and behavioral data to further explicate the mechanisms underlying attention problems in children with ADHD. These findings will likely have important implications for the refinement of existing interventions based on distinct neurodevelopmental profiles and will lay the foundation for incorporating multiple levels of analysis (i.e., neurobiological, neurocognitive, behavioral) into a precision medicine approach to treatment of the disorder.